1. Field of the Invention
This invention relates to a catalyst for purifying exhaust gases. More particularly, it is a catalyst which is suitable for purifying exhaust gases discharged by an internal combustion engine, or the like, having a high temperature and resulting from the combustion of a fuel-air mixture containing an excess of oxygen.
2. Description of the Related Art
It is important from the standpoint of energy saving to reduce the consumption of fuel in an internal combustion engine for an automobile, etc. This has hitherto been achieved by, for example, burning a lean fuel-air mixture containing an excess of oxygen on driving. The use of a lean fuel-air mixture has, however, been found to bring about a serious problem associated with the combustion product thereof. Its combustion product contains various harmful substances, such as HC, CO and NO.sub.x. Although HC and CO can be removed by oxidation, the removal of NO.sub.x, which need be accomplished by reduction, is difficult, since the contact of NO.sub.x with a catalytically active element for purifying exhaust gases is greatly hindered by the oxygen which has been adsorbed to the catalyst bed. Even if NO.sub.x may satisfactorily contact the catalytically active element and be reduced to nitrogen, the nitrogen immediately combines with the oxygen in the catalyst bed. Thus, it has hitherto been possible to achieve only a very low degree of efficiency in the catalytic purification of exhaust gases, as far as the removal of NO.sub.x is concerned.
U.S. Pat. No. 4,297,328 discloses a catalyst developed to solve the problem as hereinabove pointed out. It comprises a support of zeolite carrying copper in the numerous pores which it has in its surface, and is used for removing NO.sub.x by reduction in an oxidizing atmosphere. This catalyst is, however, unacceptable for bility of zeolite.